The present invention relates generally to linear motors. More particularly, the invention relates to three-axis linear motors used in semiconductor processing apparatuses.
Exposure apparatuses for semiconductor processing are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that positions a reticle, an optical assembly, a wafer stage assembly that positions a semiconductor wafer, a measurement system, and a control system. For many exposure apparatuses, space is often at a premium. Thus, it is often desirable to make many of the components of the exposure apparatus as compact and efficient as possible.
One type of stage assembly includes a stage base, a stage that retains the wafer or reticle, and one or more movers that move the stage and the wafer or the reticle. One type of mover is a linear motor that moves the stage along a single axis. Because it is often necessary to move the stage along more than one axis, multiple linear motors are typically required to move the stage with more than one degree of freedom. These multiple linear motors can complicate the design of the stage assembly and occupy a significant amount of space in the exposure apparatus.
A common three phase linear motor includes a pair of spaced apart magnet arrays that are surrounded by a magnetic field and a conductor array positioned between the magnet arrays. A three phase electrical current is directed to the conductor array. The electrical current supplied to the conductor array generates an electromagnetic field that interacts with the magnetic field of the magnet arrays. This generates a controlled force that can be used to move the conductor array relative to the magnet arrays along a first axis.
Unfortunately, electrical current supplied to the conductor array also produces uncontrolled forces along a second axis (shown as the Z axis in the figures) that is orthogonal to the first axis, along a third axis (shown as the X axis in the figures) that is orthogonal to the first and second axes, and about the second axis. These forces can cause disturbances that are transferred to other components of the exposure apparatus and positional error. Such linear motors are typically sinusoidally commutated brushless motors with a magnetic flux distribution, which also is sinusoidal. In general, in such motors, the endturns of coil windings in the conductor array lead to no useful force production and as such only contribute to the power losses in the motor. Therefore, it would be desirable to minimize such power losses and to utilize the endturns of the coil windings in a useful manner to provide motion along a second or third axis. By utilizing the endturns to produce useful force, one also makes use of the space available in a more efficient manner.